1. Field of the Invention
The invention relates to a configuration of a back surface of a body in a differential pressure measuring apparatus having a pre-loading diaphragm. The invention also relates to the differential pressure measuring apparatus having the pre-loading diaphragm.
2. Description of the Related Art
JP-UM-A-S60-181642 (pp. 2–5, FIG. 1) is referred to as a related art of the differential pressure measuring apparatus.
FIG. 1 is an explanatory view showing the configuration of an example of a differential pressure measuring apparatus as a related art.
In FIG. 1, the reference numeral 1 designates a body. The body 1 includes a columnar neck 1A and a block-like pressure detection assembly 1B connected by welding to a welding part 1C in the outer circumferential edge portion of an end portion of the neck 1A. In this case, each of the neck 1A and the pressure detection assembly 1B is made of a stainless steel material.
A high pressure-side flange 2 and a low pressure-side flange 3 are fixed to the opposite sides of the body 1 by welding or the like. A high pressure-side induction hole 4 for high-pressure fluid with high pressure-side pressure PH to be measured and a low pressure-side induction hole 5 for low-pressure fluid with low pressure-side pressure PL to be measured are provided in the flanges 2 and 3 respectively.
A pressure measurement room 6 is formed in the body 1. A center diaphragm 7 and a silicone diaphragm 8 are provided in the pressure measurement room 6.
The center diaphragm 7 and the silicone diaphragm 8 are fixed to walls of the pressure measurement room 6 separately so that the pressure measurement room 6 is divided into two by the center diaphragm 7 and the silicone diaphragm 8.
Back surfaces 6A and 6B are formed in walls of the pressure measurement room 6 opposite to each other with respect to the center diaphragm 7. The center diaphragm 7 is welded to the body 1 at its circumferential edge portion.
The silicone diaphragm 8 as a whole is made of a monocrystalline silicone substrate.
Impurities such as boron are selectively diffused into one surface of the silicone substrate so as to form four strain gauges 80, while the other surface of the silicone substrate is machined and etched so as to form a concave diaphragm 8 as a whole.
When the silicone diaphragm 8 bends due to a differential pressure ΔP applied thereto, two of the four strain gauges 80 are expanded and the other two are compressed. These four strain gauges 80 are connected to a Wheatstone bridge circuit so that a change in resistance is detected as a change in the differential pressure ΔP.
By the silicone diaphragm 8, the neck 1A is divided into two sensor rooms 81 and 82.
The silicone diaphragm 8 is bonded and fixed to an end surface of a sensor holding 9 on the pressure measurement room 6 side by a method such as connection using low-melting glass.
High pressure-side and low pressure-side conducting rooms 10 and 11 are formed between the body 1 and the high pressure-side flange 2 and between the body 1 and the low pressure-side flange 3, respectively.
High pressure-side and low pressure-side seal diaphragms 12 and 13 are provided in the high pressure-side and low pressure-side conducting rooms 10 and 11 respectively. Back surfaces 10 and 11A each having a similar shape to that of the corresponding seal diaphragm 12, 13 are formed in walls of the body 1 opposite to the seal diaphragms 12 and 13.
High pressure-side and low pressure-side seal diaphragm rooms 12A and 13A are formed between the seal diaphragm 12 and the high pressure-side back surface 10A and between the seal diagram 13 and the low pressure-side back surface 11A, respectively.
The seal diaphragms 12 and 13 are welded to the pressure detection assembly 1B at their circumferential edge portions by seal rings 121 and 131.
In this case, each of the seal diaphragms 12 and 13 and the seal rings 121 and 131 is made of a stainless steel material.
The seal diaphragm rooms 12A and 13A are made to communicate with the pressure measurement room 6 through conducting holes 14 and 15 respectively.
The seal diaphragm rooms 12A and 13A are filled with oils 101 and 102 such as silicone oils so that the oils 101 and 102 reach the upper and lower surfaces of the silicone diaphragm 8 through high pressure-side and low pressure-side oil transfer holes 16 and 17.
The oils 101 and 102 divided into two by the center diaphragm 7 and the silicone diaphragm 8 are arranged so that the amounts of the oils 101 and 102 are substantially equal to each other.
In the aforementioned configuration, when pressure acts from the high pressure side, the pressure applied to the high pressure-side seal diaphragm 12 is transmitted to the silicone diaphragm 8 by the oil 101.
On the other hand, when pressure acts from the low pressure side, the pressure applied to the low pressure-side seal diaphragm 13 is transmitted to the silicone diaphragm 8 by the oil 102.
As a result, the silicone diaphragm 8 is deformed in accordance with a difference between the pressure on the high pressure side and the pressure on the low pressure side. The deformation amount of the silicone diaphragm 8 is electrically extracted by the strain gauges 80. In this manner, differential pressure is measured.
“f(φ)” expressing the characteristic of an excluded volume “ΔV” of the diaphragm with respect to the pressure “P” applied thereto is used as a characteristic index of the center diaphragm 7 used in the differential pressure measuring apparatus as an example in the related art shown in FIG. 1.
A linear relation having a gradient as constant as possible in a measured pressure range is requested as the characteristic.
A differential pressure gauge using the center diaphragm 7 with such an ideal characteristic can obtain an output signal from a sensor in proportion to process pressure. Therefore, a measurement error can be reduced to the utmost even if a high-order item of a high-order equation for signal conversion in an amplifier and arithmetic operation for correction in a CPU is omitted.
That is, the differential pressure measuring apparatus in a three-diaphragm system used heretofore, that is, constituted by three diaphragms in total (i.e. two high/low pressure-side seal diaphragms 12 and 13 in contact with the process sides and one center diaphragm 7) has a structure that the center diaphragm 7 moves to any one of the opposite sides by the process pressure.
As a result, it is necessary to obtain a constant linear characteristic “f(φ)” in a wide range where the diaphragm 7 can move to any one of the opposite sides.
In order to obtain such characteristics, a solution by increasing the outer diameter of the center diaphragm 7 or by reducing the thickness of the center diaphragm 7 is effective.
However, the reduced thickness results in increase of generated stress. The solution by increasing the outer diameter of the center diaphragm 7 is mainly used.
In the differential pressure measuring apparatus using the three-diaphragm system, when overpressure is applied to the high pressure-side or low pressure-side seal diaphragm 12 or 13, the seal diaphragm applied with the overpressure is brought into close contact with the body 1.
In this manner, the structure is made such that the oil 101, 102 does not move and no pressure is transmitted to the sensors 80 so that overpressure is not transmitted to the sensors 80.
When a pre-loading type center diaphragm having a load given thereto in advance is used as the center diaphragm 7 in the three-diaphragm system described in the related art, stress generated on the center diaphragm 7 is increased so that a high-strength material has to be used as the material of the center diaphragm 7.
Ductility has to be secured in the material per se so as to mold the material as the diaphragm. When the stress generated on the center diaphragm 7 used in the differential pressure measuring apparatus is increased, the center diaphragm 7 per se has to be made of a non-ductile strong material.
From the above description, the material of the center diaphragm 7 used in the related art has to be a high-strength material which is hard and not ductile. However, it is difficult to mold the high-strength material as the center diaphragm 7. It is therefore difficult to solve the antithetic problems.
Accordingly, an improvement of the wavy shape of the center diaphragm is required. That is, the center diaphragm is improved to be shaped so as to increase an excluded volume, which is a differential volume between a volume in the initial state of the center diaphragm and a displaced volume at the time of applying pressure to the center diaphragm, in spite of small stress.